The present invention, in some embodiments thereof, relates to OFDM communication with rate adaptation and, more particularly, but not exclusively, to OFDM communication with multi-dimensional rate adaptation at millimeter wave frequencies.
Millimeter wave communications operate in the Extremely High Frequency (EHF) radio frequency band of 30 to 300 gigahertz (GHz). Compared to lower bands, terrestrial radio signals in the millimeter-wave (mm-wave) band are extremely prone to atmospheric attenuation, such as rain fade and humidity. Due to the small wavelength at these frequencies, modest size antennas may have a small beam width. Therefore, outdoor mm-wave communication systems often operate over line-of-sight (LOS) channels
OFDM is a frequency-division multiplexing scheme utilizing digital multi-carrier modulation. In OFDM the total frequency band is subdivided into several parallel data streams, using closely-spaced orthogonal sub-carriers to carry data. Each of these sub-carriers may be modulated independently. However, OFDM suffers from decreased robustness to phase noise (PN) and high Peak-to-Average Power Ratio (PAPR) relative to single-carrier techniques.
One of the advantages of OFDM is its ability to overcome the multi-path propagation associated with the wireless channel. OFDM has had no perceived advantages for LOS channels which do not suffer from multi-path propagation difficulties. Since OFDM has PN and PAPR disadvantages, mm-wave systems typically use single-carrier modulation techniques such as BPSK, QPSK and OOK.
Outdoor links present additional difficulties. Outdoor links should work even during worst case weather and interference conditions. One current solution for maintaining the link during worst case conditions is to operate with extreme modulation parameters such as: minimum bandwidth, low modulation, excessive error correction code and some repetitions. Such parameters yield very low throughput compared to the throughput that can be achieved in better conditions which exist most of the time. Additionally, the complex error correction required during poor channel conditions places an excessive burden on the computing power of the modem.
Rate adaptation is a technique used for overcoming changes in channel conditions. Rate adaptation dynamically adjusts the modulation parameters, such as the modulation type, coding and other signal and protocol parameters, according to the current transmission quality on the radio frequency (RF) channel. However, implementing rate adaptation in OFDM communications is particularly difficult due to the large number of modulation parameters (i.e. degrees of freedom) which can be dynamically controlled.
The Flexport Ethernet Wireless Links, by BridgeWave Communications Inc., utilize QPSK/BPSK modulation with adaptive modulation. In mm-wave systems the RF channel may be hundreds of megahertz (MHz) wide. Conventional modulation schemes, such as QPSK and BPSK, introduce dispersion into the signal passed through the channel. When operating over a very wide channel, QPSK/BPSK systems have difficulty maintaining the channel flat in frequency and phase.
U.S. Pat. No. 7,164,649 to Walton et al. presents techniques to adaptively control the rate of a data transmission in a wireless (e.g. OFDM) communication system. Various metrics may be used to select the proper rate for the data transmission, such as different characteristics of the communication channel or the performance of the data transmission.
“Millimeter-wave OFDM WPAN system applying adaptive modulation for grouped sub-carriers” by Shoji et al. (Radio and Wireless Symposium, 2007 IEEE, Jan. 9-11, 2007) presents a millimeter-wave OFDM modulation technique for an indoor channel.